The generation of ions for storage in mass-spectrometric ion traps is dependent on the concentration of the substances to be ionized. The ion trap mass spectrometer is, as are other mass spectrometers, frequently coupled to chromatographic processes of separation which naturally cause extreme fluctuations in the flow of carrier gas. However, methods which produce substance vapors in bursts, such as pyrolysis or evaporators, also produce extreme fluctuations in concentration.
If ion traps are used as mass spectrometers, the maximum number of ions which can be stored at any one time must not go beyond a very sharply defined limit or else the mass spectrum will deteriorate in two respects:
Firstly, the mass lines of the spectrum compared with a correct calibration are displaced by more than a few tenths of an atomic mass unit; and
Secondly the mass lines become wider as mass resolving power declines.
The reason for these effects is the ion-generated space charge which impairs the functioning of the ion trap.
On the other hand, the number of ions which are available for measuring a spectrum below the space-charge limit is relatively low. Depending on the type of ion trap there are only about 1,000 to 10,000 ions available per spectrum for measuring the entire spectrum with all its mass lines. Consequently the dynamic range of measurement within a spectrum is very small and is only scarcely 2 to 3 orders of magnitude. For scanning a mass spectrum, however, measurement of weak mass lines down to 0.1% is normal, which is usually only successful in ion traps if a number of spectra are added together. Even in such a case, precision can not be expected to be good for measuring the weak mass lines. The dynamic range is still barely adequate to measure two substances which are inside the ion trap at the same time and which have different concentrations.
For this reason it is necessary to optimally utilize the maximum number of ions before the space-charge limit is reached.
As already known from the similar case of ion cyclotron resonance mass spectrometry (ICR), it is useful to control the generation of ions so that the spacecharge limit is just not reached.
For this type of control a variable must be measured which is representative of the space charge (or rather, of the number of ions stored), and which can be used for automatic control purposes. As the considerable fluctuations in concentration cannot be forecast quantitatively, it has proved to be a reasonable aim to control a tolerance interval which is approximately between the space-charge limit itself and a value which is about 20% below the space-charge limit. For this it is necessary to accurately know the generation rate of ions at the time of ionization for scanning to within about 10%.
Automatic control of the number of ions is already known for ion traps. U.S. Pat. No. 5,107,109 describes the type of control system for generating the items by electron impact in ion traps, and U.S. Pat. No. 4,771,172 describes an equivalent control system for chemical ionization. In both cases, generation of the ions for measurement of the spectrum is initially preceded in a preliminary phase by measurement of ion generation rate. In the preliminary phase an initial ionization takes place with a short, constant ionization time under constant ionization conditions. After a deceleration time for the ions created in which they collect at the center of the ion trap, the ions thus generated in the preliminary phase are ejected from the ion trap in a brief ejection process and measured in an integrating process. Using the quantity of ions thus measured in the preliminary phase, an ionization time is then calculated which produces an optimal number of ions in the ion trap for the subsequent scanning phase. The ion trap is then completely emptied until the preliminary phase is terminated. It is reset and then filled with ions in the second ionization process proper for the scanning phase.
European Patent EP-B 10 237 268, which is based on the priority of the application of U.S. Pat. No. 5,107,109, even places automatic control of the space charge in ion traps as such under protection without any specific reference to a measurement of the actual values, and not only the method of preliminary phase measurement of the claim granted in U.S. Pat. No. 5,107,109.
Control of the ionization process resulting from automatic control of space charge is, in practice, usually related to the duration of ionization, whereby the intensity of ionization is kept constant. In the case of electron impact ionization the electron beam is kept constant and the time the electron beam is allowed to act on the substance is limited by an electron beam switch (shutter). Control of duration can easily extend over a wide range and in practice it covers approximately 3.5 powers of ten from 5 microseconds to 20 milliseconds. Although it would be possible to control the intensity of the electron beam as well, it would be difficult and this has so far not been applied.
Automatic control of the number of ions in ion traps by measuring the ion generation rate beforehand has produced a significant improvement in the spectra from chromatographic separations. Displacement of the mass lines was kept within limits and the mass resolving power largely remained constant. However, measurement of the generation rate in a preliminary phase still has considerable disadvantages in very fast chromatography.
Between generation of the ions in the preliminary phase and generation of the ions for the scanning phase there are about 10 milliseconds. Activites to be perfomed within this time include, consecutively, ion deceleration, ion ejection with measurement, emptying of the ion trap, and resetting. On the other hand, the concentration can already change easily by a factor of 2 in 10 milliseconds if fast chromatography is used with narrow peaks. In the case of chemical ionization the relationships are much less favorable because the time between the two ionization phases is much longer.
Also, in the preliminary phase the space-charge density is naturally not controlled. However, the levels of concentration can easily change in a chromatogram over 4 to 6 powers of ten (measured above the noise background). Depending on the prevailing concentration, the number of ions formed in the preliminary phase can be so small that measurement of the generation rate has a large degree of uncertainty. On the other hand, the number of ions formed may be so large that the space-charge limit is already considerably exceeded and the ejection process, and hence measurement of the generation rate, is already impaired. In both cases an incorrect or uncertain value for ion generation rate impairs calculation of the optimal ionization time for the subsequent ionization phase for scanning.
Therefore, It is among the objects of the present invention to control generation of the ions in an ion trap used for mass spectrometry in such a way that an optimal number of ions is formed and stored below the space-charge limit. As used herein, the term "space-charge limit" means the number of ions above which a considerable deterioration in spectra can be observed. This number of ions can be defined in a preceding calibration process. In particular it should be possible to accurately control the ions stored for scanning to within a few percent, even if there are considerable temporal changes in substance concentrations, as occur in fast chromatography.